Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare read-only/open-world behavior, so the description's added value is the payment context: $0.003 USDC per call via x402 or prepaid credits, plus the free-trial note and the authoritative ONS/postcodes.io source. That monetization and sourcing detail is genuine behavioral context the annotations do not carry.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Three tight sentences, front-loaded with the core lookup and its outputs before price and provenance. The dense field list earns its place; only the trailing 'In the free trial.' fragment is slightly dangling.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

An output schema exists, yet the description still usefully summarizes the returned fields, and it discloses cost/auth up front. The one real gap is guidance on the postcode-versus-lat/lon modes, which an agent must infer from the schema alone.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100% and every parameter is annotated in-schema, including the lat/lon semantics and the 'spaces optional' note for postcode. The description adds no parameter-level detail beyond what the schema already provides, so the baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb+resource ('UK postcode lookup') and enumerates the returned data (coordinates, region, ward, LSOA/MSOA, ONS codes), so an agent knows exactly what it gets. It does not, however, distinguish itself from the nearby uk-area sibling, which sounds like it could overlap.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

No when-to-use, when-not-to-use, or alternative guidance is given. The description never explains when to supply a postcode versus lat/lon for a nearest-postcode search, even though those are two distinct invocation modes.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.